In balancing machines for measuring and correcting the dynamic balance of rotary parts such as shafts, it is the usual practice to rotate the part, measure the phase and amplitude of any vibration which results from unbalance of the part, compute the amount and position of weight additions required to compensate for the unbalance, attach the weights, and make another balance measurement to verify that the part is corrected for unbalance. Often the weight attachment is effected by electrical welding.
To accommodate the vibration and allow its measurement, the balancing machine base structure is provided with a flexure region or suspension so that a portion of the machine vibrates with the rotating part and the vibration of the machine portion (which is otherwise stationary) can be measured to determine the part unbalance. Where the balancing machine has a stiff suspension, the amplitude of vibration to be measured may be very small, say, 2*10.sup.-6 inch. Thus it is required that the vibration sensing device be very sensitive, accurate, and linear as well. While a number of sensing devices have been used in the past, one which is particularly well suited to provide the listed requirements as well as repeatability and ruggedness is an eddy current proximity detector. Such a detector will reliably measure the vibration of the machine.
It has been found, however, that the welding procedure has the effect of magnetizing the part and even the machine tooling which supports the part. During part rotation, the residual magnetic field, in turn, causes the eddy current proximity detector to generate a signal which is indistinguishable from a vibration signal and thus yields erroneous readings. Since the magnetic field as well as the unbalance vibration varies with the rate of rotation, the magnetically induced signal has the same frequency as a vibration signal. Moreover, the amplitude of the spurious signal is in the range of measured vibration signals. Thus it is desirable to eliminate any confusion or error which may result from the magnetically induced signals.